When transient sounds are presented to human subjects, the summed response from many remotely located neurons in the brain can be recorded via non-invasive electrodes (e.g. attached to the scalp and/or located in an ear canal of a person). These auditory evoked potentials (AEPs) can be recorded from all levels of the auditory pathway, e.g. from the auditory nerve (compound action potential, CAP); from the brainstem (auditory brainstem response, ABR); up to the cortex (cortical auditory evoked potential, CAEP), etc. These classical AEPs are obtained by presenting transient acoustic stimuli at slow repetition rates. At more rapid rates, the responses to each stimulus overlap with those evoked by the preceding stimulus to form a steady-state response (as defined in [Picton et al., 1987]). In early studies, such auditory steady-state responses (ASSR) were also evoked by sinusoidally amplitude modulated (AM) pure tones. Due to the tonotopic organisation of the inner ear (the cochlea) and auditory pathway, the carrier frequency for AM tones determines the region of the basilar membrane within the cochlea being excited, but producing evoked responses that follow the modulation frequency. In this way, ASSR has proved to be an efficient tool for testing different frequency locations within the auditory pathway. AM tones are the simplest frequency specific stimuli used to evoke ASSRs, but they only stimulate a small number of auditory nerve fibres resulting in relatively small response amplitude. This small response can be problematic for response resolution, so various methods have been developed to increase area of excitation in the cochlea to recruit more auditory nerve fibres, and to increase response amplitude and hence response detection and accuracy.
WO2006003172A1 (U.S. Pat. No. 8,591,433 B2) describes the design of frequency-specific electrical or acoustical stimuli for recording ASSR as a combination of a series of multiple spectral components (i.e. pure tones), designed to optimise response amplitudes. This was achieved by pre-compensating for the frequency-dependent delay introduced in the inner ear (cochlea) to achieve more synchronised auditory nerve firing across frequency. It is a basic characteristic of an auditory evoked response that the magnitude of the response depends on the number of auditory units/nerve fibres that are activated synchronously by the stimulus (as defined in [Eggermont, 1977]). By compensating for the inherent cochlea frequency-dependent delay, and specifically defining the magnitude and phase of each of the multiple spectral components, a repetitive frequency glide or chirp can be created. The repetition rate of this chirp-train is determined from the frequency spacing between the spectral components used in its generation, and the band-width is controlled by the number of components chosen. In this way, a very flexible and efficient stimulus for recording ASSR can be created (cf. e.g. [Elberling, 2005], [Elberling et al., 2007a], [Elberling et al., 2007b] and [Cebulla et al., 2007]).
One of the major advantages of ASSR is the ability to perform simultaneous multi-band recordings, i.e. to present multiple stimuli at different carrier frequencies to both ears—each with different repetition rates, and hence detection and observation of these potentials are typically made in the frequency domain. The stimulus and response structure in the frequency domain are well-defined and importantly predictable, thus ASSR lends itself well to automatic detection algorithms based on sets of harmonics of the modulation/repetition rates.
At present, ASSRs in the clinic and in research are stimulated using repeated trains of broad-band and narrow-band chirps, amplitude modulated tones, combined amplitude and frequency modulated tones, and trains of clicks and tone-bursts. As a result of highly successful universal new-born screening programs in many countries, paediatric audiologists are now routinely seeing patients within the first-few weeks of life (Chang et al., 2012). Therefore, it is advantageous to have hearing aid fitting protocols designed for young infants, as the sooner the intervention then the better the clinical outcomes. Thus the use of ASSR for estimating audiometric threshold is fast gaining ground for use with neonates referred from a screening programme. Determining these patients' thresholds via behavioural measures is highly unreliable or impossible, hence the need for objective physiological methods. Regression statistics exist to calculate the expected difference between physiologic and behavioural thresholds at each sound level. Accurate threshold estimation depends on being able to determine whether a small response exists in the presence of residual background noise. In addition to infants, ASSR and objective measures are used with hard to test adults, i.e. adults with severe mental or physical impairment.
Once a hearing aid is fitted to a particular user, then parameters need to be adjusted, for example to ensure that gain is set such that the speech spectrum is amplified within the dynamic hearing range of the subject. For good practice, a verification of this fitting needs to be made to ensure that this is in fact the case. A robust objective method for doing this is important. Sound field auditory evoked potentials—ABR, CAEP and ASSR have all been proposed as potential methods for doing this. It has been shown that ABRs are inappropriate, as the stimuli used to evoke them are typically very short (<10 ms), and hearing aid processing distorts the stimulus, making the response waveform hard to interpret.
CAEPs are growing in popularity for verification of hearing aid fitting. In particular, CAEPs evoked from short-duration phonemes and speech-like stimuli are argued to reflect neural encoding of speech and provide objective evidence that the amplified speech has been detected. CAEPs have documented disadvantages that they are strongly affected by the attention of the subject, which is hard to control in infants. Also, objective detection of response waveforms is challenging as the waveforms/evoked potentials vary significantly across subjects. Finally, even though they are longer in duration than ABR stimuli, typical stimuli to evoke CAEPs are still relatively short, and hence are subject to the same distortion and disadvantage as described above.